Phosphate recovery from an aqueous solution through adsorption-desorption cycle over thermally treated activated carbon

In the present study, we tested nine commercially available activated carbons for their abilities to recover phosphate ion from an aqueous solution by a temperature swing method, in which phosphate ion was adsorbed at 303 K, followed by desorption and recovery in pure water at 373 K. While the activated carbon made from coconut shell and manufactured by Nakarai Tesque Inc. (trade name: Charcoal Activated) had a moderate adsorption capacity at 303 K, little amount of phosphate ion was adsorbed on it at 373 K, meaning that it was an appropriate adsorbent for the temperature swing method. Since the adsorbed amounts of phosphate ion for various activated carbons at 303 K were correlated with the number of basic sites on them and were significantly increased as pH of the solution decreased, it is presumed that phosphate ion was adsorbed on the basic sites of the activated carbons with electrostatic interaction mediated by protons. High-temperature thermal treatment of the activated carbon in a vacuum increased the recovered amount of phosphate ion. This increase was brought about by elimination of oxygen-containing functional groups from the activated carbon. By using activated carbon obtained by the thermal treatment at 1273 K for 3 h, 86 of phosphate ion was recovered from an aqueous solution with 1.0 mmol L–1 of phosphate ion through the temperature swing between 303 and 373 K. The thermally treated activated carbon was reusable at least three times without any severe performance loss.